The present invention relates to improvements in an apparatus for controlling the drive of an elevator by an inverter control system.
Heretofore, there has been an elevator in which an induction motor is used as a hoisting electric motor for elevating upward or downward a cage, the induction motor being controlled to be driven by a drive controller employing an inverter.
Such a drive controller for an elevator is disclosed, for example, in the official gazette of Japanese Patent Application Laid-open No. 60-2075, and is shown in FIGS. 5 and 6.
In FIG. 5, this drive controller inputs a three-phase A-C output from a three-phase A-C power source 1 through a switch 2 to a power converter 3, and the power converter 3 converts the three-phase A-C output into a D-C output.
The D-C output of the power converter 3 is smoothed by a capacitor 4, the smoothed output is input to a transistor inverter 5 of a power inverter, the three-phase A-C output converted from the D-C output by the inverter 5 is applied to a hoisting motor 6 formed of an induction motor to drive the hoisting motor 6.
Since a sheave 7 connected to the rotational shaft of the hoisting motor 6 is thus rotated, a cage 10 is elevated upward or downward by a hoisting rope 9 wound on the sheave 7 and coupled at one end with a balance weight 8 and at the other end with the cage 10.
A command speed signal P.sub.A from a pattern generator 11 for generating the speed pattern for the cage 10, a detection speed signal P.sub.B from a tachometer generator 12 for detecting the rotating speed of the hoisting motor 6, i.e., the running speed of the cage 10 and a detection current signal I.sub.A of a feedback signal from a current detector 13 for detecting the three-phase output current of the inverter 5 are input to a regulator 14 of a power inverter controller.
Thus, the regulator 14 alternatively base-drives the transistors of the inverter 5 in accordance with the input signals P.sub.A, P.sub.B, I.sub.A to control A-C to D-C conversion, thereby controlling the rotating speed of the hoisting motor 6, i.e., the running speed of the cage 10.
The regulator 14 has, as shown, for example, in FIG. 6, a current commanding circuit 15 for comparing the command speed signal P.sub.A from the pattern generator 11 with the detection speed signal P.sub.B from the tachometer generator 12 to generate a command current signal I.sub.B, a subtractor circuit 16 for comparing the command current signal I.sub.B with the detection current I.sub.A from the current detector 13 to output the difference value thereof, an amplifier 17 for amplifying the output of the subtractor, a pulse-width modulator (PWM) circuit 18 for pulse-width-modulating the output of the amplifier 17 to output a pulse-width-modulation pulse, and a base driver circuit 19 having transistors controlled ON, OFF by the output pulse of the PWM circuit 18 to output a base drive signal P.sub.C to the inverter 5.
The cage 10 must be controlled to provide speeds which change smoothly in a wide range from a start to a stop operation, and the controlling characteristics of the cage largely depends upon the detecting accuracy of the current detector 13.
The current detector 13 generally outputs a small signal even if the detection current is "0". In other words, the current detector has an offset. Therefore, a command in which the offset is always applied thereto or subtracted therefrom is applied as a control signal of the inverter 5 to reduce the desired controlling characteristics. Thus, the cage 10 vibrates to deteriorate the riding comfort in the cage.
In order to eliminate the above-mentioned drawbacks, the offset of the current detector 13 is regulated to obtain the best controlling characteristics, but the offset amount is varied according to temperature changes or a longterm drift due to the characteristics of the current detector 13. Thus, since the offset of the current detector had to be regulated at every regulation, it was difficult to obtain a stable controll characteristics over a long period. Since the offsets of all the elevators must be regulated at the time of installing the elevators, there also arise drawbacks in the installation and the maintenance of the elevators.